Method of tracing the flow of liquids by use of post radioactivation of tracer substances



Sept. Z6, 1961 F. E. ARMSTRONG METHOD OF TRACING THE FLOW OF' LIQUIDS BYUSE OF POST RADIOACTIVATION OF' TRACER SUBSTANCES Filed Nov. s, 1958ATTORNEY 3,002,091 METHOD OF TRACING THE FLOW F LIQUIDS BY USE OF POSTRADIOACTIVATION OF TRACER SUBSTANCES Frederick E. Armstrong,Bartlesville, Okla., assigner to the United States of America asrepresented by the Secretary of the Interior Filed Nov. 3, 1958, Ser.No. 771,683 8 Claims. (Cl. Z50- 83) (Granted under 'ltle 35, U.S. Code(1952), sec. 266) The invention herein described and claimed may bemanufactured or used by or for the United States of America forgovernmental purposes without payment of royalty thereon or therefor.

This invention relates to a method for determining the how of fluidsthrough porous media, e.g., earth layers, and tracing their pathstherethrough by injecting into a borehole a duid containing an element,which on neutron irradiation forms a radioisotope, removing the iiuidfrom another borehole spaced from the rst, irradiating the effluentfluid with a neutron source, and detecting the radioactivity produced inthe added element.

In the production of oil and gas, knowledge of the rate of flow of uidswithin and through the subsurface media, particularly during stimulativeinjection of water or gases into a petroleum reservoir, is of greatimportance. Choice of the method of secondary recovery is greatlyfacilitated by knowledge of the direction, rate, or extent of ow ofinjected iluid into an earth formation. The problem is to determinewhether the injected tiuid is taking the desired path through the oilsands, thereby displacing oil from the sand and forcing it to movetoward a producing well or wells, whether the injected iluid isbypassing the oil sands through fractures or other nonhomogeneousconditions, or whether it is flowing by other paths away from producingwells.

Various methods have been proposed for obtaining this information. Onescheme proposes adding a boron compound to injection water and detectingthe boron spectrographically at the producer well. Another methodproposes the addition of -acetylene to the injection fluid and detectingit at the producing well by sensitive chemical tests. In still anothermethod, it is proposed to -admix a radioactive element with theinjection fluid and then sensing its presence in the eliiuent at theproducing well by a detector for radioactive materials, e.g., aGeiger-Mller or a scintillation counter.

Methods relying on spectrographic or chemical analysis requirerelatively large amounts of tracer material, While the former method isfurther handicapped in requiiing an emission spectrograph, an instrumentwhich is not readily adaptable for use in the field. Radioisotopes aregenerally comparatively expensive, and considerable health hazardattends their use. Most important, suitable radioactive isotopes whichhave ya half-life suiciently long to enable them to be used in tracertests that may extend over several months are not common. Many dyes havealso been tried, but have been found to be strongly absorbed by thereservoir material and hence unsuitable.

It is an object of this invention to provide a method for tracing the owof iluids through porous media by the addition of a material to thefluid, which is not easily absorbed by the media and Iwhich may be maderadioactive at the point of detection.

It is a further object to provide a method for tracing the flow ofliuids through porous media by injecting a measured quantity of aharmless element into the iluid, irradiating the fluid at a point ofdetection with a neutron source to form a radioisotope, and detectingand quantitatively measuring the radioactive material.

Other advantages will appear in the following description of myinvention and the appended claims.

Patented Sept. 26, 1961 A preferred mode which has been contemplated forapplying the principles of the invention, is illustrated by the figureof the drawing showing a flow diagram comprising structures and devicthrough the use of which the method of the invention may beaccomplished.

With reference to the igure of the drawing, this invention consideredbroadly, is shown to relate to an improved method for studying the uidconductance of subsurface strata 14 by impressing a uid containing aharmless amount of element capable of yielding radioactive material whenirradiated with neutrons. This iluid is supplied from a container 1, andis impressed into a test borehole of an injection well 3 intersectingthe strata and recovered at a recovery lborehole or well. The effluentuid is irradiated with neutrons from a suitable' source 7 and theresultant radioactivity is quantitatively detected by well knowndetection means y11. Since the material is not activated until it isnear the point of detection, elements which yield short half-liferadioisotopes can advantageously be employed. This is important from ahealth standpoint, in View of the problems of storage, and wastedisposal. The flow of potable waters can be traced by these means, sincevery small quantities of non-toxic compounds can be employed as thetracing means. With initially radioactive tracers this is not desirabledue to danger of radioactive pollution.

Radioisotop that have been employed for tagging or tracing uid ilow areusually produced by irradiation by neutron fluxes of high densities in anuclear reactor.

Most elements require irradiation at flux densities of l08 or 109neutrons per cm2 per second for some time to produce enoughradioactivity to permit the use of the irradiated product as a tracer.There are about 24 elements known at present, which yield detectableradioactivities with irradiation times as short 'as 'a few minutes atflux levels of about 107 neutrons per cm.2 per second. Nine of these;dysprosium, europium, vanadium, rhenium, silver, indium, scandium,iodine, iridium, give exceptionally high yields of radioactivity. Inmost instances the half lives of the produced isotopes are very short,some in the order of minutes.

In one embodiment of my invention, the pattern and rate of now betweeninjection wells such as shown by element 3 of the drawing, and producingWells such as shown by element 4, of a petroleum Water-flood system isdetermined by rst adding a quantity of the element which is to be usedas the tracer of the input iluid, usually water or brine in waterflooding. The element employed is in the form of a water soluble salt,and is one which does not occur normally in the underground brine orwater in the oil field. Dysprosium, iridium, europium, and scandium, andiodine are satisfactory, but of course the other elements may beemployed where desired. The amount to be yadded depends on the degree ofdilution expected before the tracer reaches the detection point.Concentrations las low as 0.1 part per million would be readilydetectable. If information is required concerning the exact degree ofdilution, then the quantity of tracer added must be known exactly.

Water containing the tracer element in solution is removed at theproducer well 4, by a sampling line 5, and passed into a vessel 6wherein it is irradiated for a suitable period of time with neutrons ata iiux density of preferably 10"l neutrons per cm.2 per second, orhigher. One suitable source of this flux density is a large sizeradiumberyllium neutron source 7, comprising an `accelerator beam tubeand target assembly such as commonly used for neutron-logging oil wells.Also, a small accelerator of the Van de Graalf type, recently introducedas a neutron source in the well-logging art, may be employed. The latterhas the advantage, in addition to producingra higher uX density than the,usualirradium-.beryllium source, that it is not a potential radiationhazard when not in operation.

The flow rate of Iliquid into and out of the irradiation vessel 46 iscontrolled so as to maintain a constant rate of irradiation of thesample. This is important since the amount of radioactivity is a'function both of the amount of Vsample present and the irradiationtime, the flow rate being held constant.

The liquid leaving the irradiation unit is radioactive to an extentdependent on the flux density of the neutrons, the amount of otherelements with high neutron-captive cross section present, the time ofirradiation and the amount of tracer present. After irradiation thetracer element may be measured in several ways. If no, or few othersubstances -with neutron-capture cross sections comparable to that ofthe tracer are present, the induced radioactivity may be determined witha Geiger-Mller counter, preferably of the rate-meter type with provisionfor recording the intensity of radioactivity. If the amount of tracerthat may be expected to be produced is small, it may be desirable toemploy a scintillation counter 11, which is more sensitive. Should aconsiderable `amount of the radiation present be caused by elements ofhigh neutron-capture cross section normally present in the iluid,counting should be done with instruments that can be made to respond tobut one energy level of radiation. The scintillation counter may beadapted to be employed in this manner, as is known to those skilled inthe art. This counting or measuring procedure permits measuring theradiation emitted by the tracer even in the presence of considerableradiation from other sources. However, if there are present largequantities of lsubstances interfering with the counting technique, asimple separation may be made, either to separate the tracer from theliquid, or remove the interfering substances from the liquid.

A gaseous tracer may be employed where the driving uid is a gas. Alsofinely divided solid tracer material which can be dispersed in the gasor liquid medium may be employed. As used in this application andclaims, the individual elements named are also intended to include theirchemical compounds, since so far as regards neutron irradiation, theyare equivalent.

ff 3,002,091 f f Example A tracer test is made in a typical waterlioodreservoir having a strata 14 of a sandstone formation 20 feet thick at600 feet, a permeability of 50 mi'l-lidarcys, a porosity of 20% and awater saturation of 80%. Water injected at the rate of 200 barrels perday into the center well such as the injection well 3, of an invertedtive-spot drilling pattern should appear at the producing wells such asthe Well 4, in 120 days if no channelling or highly permeable streaksbetween input and producing wells exist. This is calculated by agraphical method from the equation assuming ideal conditions, and theequipressure contours and stream travel lines for two wells of ahomogeneous S-spot pattern as described by Wycoff, Bolset and Muscat involume 103, Transact AIME (1933). In the equation:

From calculations based on the above equation and familiar to thoseskilled in the art, it can be shown that tracer injected for a period of20 hours at the injection well 3, into a total of approximately 170barrels of injected water, should produce a minimum dilution ofapproximately 10 times at the surrounding producing wells. Accordingly,an amount of tracer is added from the supply 1 of tracer material insolution, by means of proportioning injection pump 2, to the injectedwater which may be expected to produce a concentration in the producedwater suiciently high to allow it to be detected by the methoddescribed. In most instances the required concentration at the producingwell will be less than .l part per million by weight. In the presentexample, 5 kilograms of potassium iodide is proportioned by the pump Zinto the injection water over a period of 20 hours, producing aconcentration at the producing wells somewhat greater than 10 parts permillion under ideal conditions. If the condition of the formationbetween injection and producing wells is other than homogeneous, thetracer material will arrive sooner and with less dilution, the amount ofdilution becoming less with further decreases in homogeneity. Comparisonof such data with the calculated data can thus indicate the amount ofnon-homogeneity which exists in the formation.

At the production well, a simple oil-water separator removes a portionof the produced water and permits it to flow through line S and thechamber of the activation unit comprising elements 6 and 7, connected toa highvoltage supply and controls 8. This preferably is a smallaccelerator of the D-T type operating at an accelerating potential ofkilovolts employing the reaction D+T He4|nl17-7 mev., such as isdescribed in Nucleonics, vol. 15, No. `9, pp. 192-194. Such a device hasan accelerating tube such as element 7, about 12 inches long, is 19/16inches in diameter, and produces a flux of about 108 neutrons persecond. The produced water flows into and through the chamber 6surrounding the target of the accelerator tube '7, thus acting as -amoderator and at the same time exposing the trace material to theneutron tiux for activation. A ow rate of about 10 milliliters perminute should allow an average exposure time of approximately l hourwith a 3 liter chamber. After activation the water Hows through a shield9 to the detector unit which consists of a Geiger-Mller thin-wall tubesuch as element 11 and a recording counting-rate meter 12. The sampleHows through a container 10 surrounding the Geiger-Mller tube with avolume essentially the same as that of the irradiation chamber and outthrough an eiuent drain 13, thus making maximum use of the sample volumeand at the same time acting as a secondary shield for the detector. Theradioisotope produced from the tracer by irradiation will be iodine 128which has a half life of Q5 minutes and emits a 2. niev. beta ray. Thechoice of the detecting element is dictated by the radiation produced bythe activated tracer element, i.e., if the radiation is primarily beta,the choice should be a Geiger tube, if gamma, a scintillation detector11 will have increased sensitivity and discrimination for this emission.

Obviously, other water soluble iodine compounds, such as inorganicsalts, acids, or soluble organic compounds may be employed. However, themost easily obtainable and economically the most feasible willordinarily prove to be the iodide salts, such as the potassium iodide ofthe example.

Other modes of applying the principle of this invention may be employed,it not being limited to the specic embodiment disclosed and the scope ofsaid invention is to be determined solely by the following claims:

I claim:

l. A process for tracing the flow of fluids underground which comprisesenriching the fluid at one underground point with a tracer elementcapable of yielding a radioactive isotope when irradiated with neutronshaving a flux density of at least 10'-l neutrons per cm.2 per second,

withdrawing a sample of thev tluid ata second underground point,irradiating said sample with neutrons having a ux density of at least Ineutrons per cm.2 per second for a period of time suicient to produceradioactive isotopes from said tracer element, and measuring theresulting radiation.

2. In a process of water-dooding an oil field and tracing the flow ofwater used in said flooding, wherein Water is injected into a producingzone in said iield through `an input well to force liquid out of atleast one production well, the steps of adding a tracer element to theinjected water, said tracer element being selected from the groupconsisting of dysprosiurn, europium, silver, rhenium, vanadium, iodine,and iridium, taking samples from one production well, irradiating saidsample with neutrons having a flux density of at least 10r1 neutrons perom? per second for a period of time suicient to produce radioactiveisotopes from said tracer element and measuring the resultantradioactivity.

3. A method for studying the characteristics of the subsurface stratawhich comprises, injecting at one underground point a gas containing atracer element, said tracer element being selected from the classconsisting of dysprosium, europium, silver, rhenium, vanadium, indium,iodine, and iridium, removing a gas sample from a point underground,spaced from the first injection point, irradiating said sample withneutrons having a ux density of at least 107 neutrons per cm.2 persecond, for a period of time sufcient to produce radioactive isotopesfrom said tracer element, and measuring the resultant radioactivity.

V4. A process for tracing the flow of fluids underground which comprisesthe steps of enriching the iiuid at one underground point with iodine asa tracer element, withdrawing a sample of fluid at a second undergroundpoint,

irradiating said sample with neutrons having a ux density of atleast10'I neutrons per cm.2 per second for a period of time suicient toproduce radioactive iodine 128, vand measuring the resulting radiation.

' 5. A process for tracing the ow of liquids underground, whichcomprises the steps of adding to the liquid at a tirst undergroundposi-tion a solution containing potassium iodide as a tracer,withdrawing a sample of liquid at a second underground position,irradiating said sample with neutrons having a flux density of lat least10'I neutrons per ern.2 per second for a period of time sufficient toproduce radioactive iodine 128, and measuring thel resulting radiation.

6. The method of claim 2, wherein the tracer added is iodine.

7, The method of claim 2, wherein the tracer added is potassium iodide.

8. The method of claim 3, wherein the tracer added is iodine.V

References Cited in the ile of this patent UNITED STATES PATENTS2,394,703 Lipson Feb. l2, 1946 2,429,577 French Oct. 21, 1947 2,443,680Herzog June 22, 1948 2,560,510 Hinson July 10, 1951 2,640,936 Pajes June2, 1953 2,841,713 Howard July 1, 1958 2,910,587 Sayre Oct. 27, 1959OTHER REFERENCES Second United Nations International Conference onPeaceful Uses of Atomic Energy, vol. 19, United Nations Press, Septemberl to 13, 1958, pages 112 to 119.

1. A PROCESS FOR TRACING THE FLOW OF FLUIDS UNDERGROUND WHICH COMPRISESENRICHING THE FLUID AT ONE UNDERGROUND POINT WITH A TRACER ELEMENTCAPABLE OF YIELDING A RADIOACTIVE ISOTOPE WHEN IRRADIATED WITH NEUTRONSHAVING A FLUX DENSITY OF AT LEAST 10**7 NEUTRONS PER CM.2 PER SECOND,WITHDRAWING A SAMPLE OF THE FLUID AT A SECOND UNDERGROUND POINT,IRRADIATING SAID SAMPLE WITH NEUTRONS HAVING A FLUX DENSITY OF AT LEAST10**7 NEUTRONS PER CM.2 PER SECOND FOR A PERIOD OF TIME SUFFICIENT TOPRODUCE RADIOACTIVE ISOTOPES FROM SAID TRACER ELEMENT, AND MEASURING THERESULTING RADIATION.